1. Field of the Invention
The present invention relates to an optical disk device. More particularly, the present invention relates to an optical disk device and an access method for the optical disk device for making reliable access while avoiding an unrecorded region during optical-head access.
2. Description of the Related Art
Recently, an optical disk device such as DVD (Digital Versatile Disc) has been developed and widely known. In such an optical disk device or the like, there is a demand for high operational reliability, usability, or the like.
As an example of such an optical disk device, in an optical disk device described in Jpn. Pat. Appln. KOKAI Publication No. 2000-251271, the following technique is disclosed. In the case of an interlayer jump of a double-layered disk, if a target address position is at the inner periphery than a current address position, a seek operation is carried out up to a radial position of the same recording layer which corresponds to the target address position. Then, an interlayer jump is carried out and reaches the target address position. On the other hand, in the case where a target address position is at the outer periphery than a current address position, an interlayer jump is carried out at the current address position, and then, a seek operation is carried out for the target address position.
In other words, in a reproduction type optical disk, an optical disk having a plurality of layers has been developed and made popular. Similarly, there is proposed a recording type optical disk having a plurality of recording layers on one side from a demand for increasing the capacity of data to be recorded in an optical disk. In the case where the disk thus has a plurality of recording layers, a transmission rate of a shallower recording layer must be increased in order to maintain the reflection index of each recording layer to be substantially constant. As a result, the reflection index to be maintained constant is reduced, and an optical disk is produced so that the reflection index of each layer is substantially equal to this low reflection index. Thus, signal levels of a variety of signals are lowered, and an S/N ratio is degraded.
On the other hand, in a recording layer, the reflection indexes of an unrecorded region and a recorded region differ depending on disk characteristics. The disk characteristics include two characteristics, i.e., a characteristic in which the reflection index is reduced by data recording as in DVD-RAM or the like and a characteristic in which the reflection index is increased by such recording. In general, a change in this reflection index is greater than twice or half. In the boundary region between a recorded region R and an unrecorded region M, a variety of signals are affected by the change in this reflection index. In particular, in the case where precise servo is required, there is a possibility that such an effect of change results in a large disturbance.
Thus, as in the conventional reproduction type optical disk device, when an interlayer jump is carried out in order to make access to a specific target position on a storage region, if the center axes of the recording layers are not aligned in a double-layered disc, an interlayer jump destination may be the unrecorded region M. In such a case, there is a problem that the disk is affected by the boundary region or a signal level which depends on the region in an unstable state after a jump, and a servo state becomes unstable.
Further, in the case of a recording type optical disk, in order to record/reproduce information, it is required that a response time from a time when a recording/reproduction request occurs to a time when a recording/reproduction operation is started is fast. Since different adjustments are required depending on the recorded region R and the unrecorded region M, it is believed that an access method in which only the recorded region R requiring minimal adjustment is passed is stable. However, as in the conventional device, according to a rectangular access method for connecting a current address position to a target address position with the shortest distance, it is not guaranteed that a region in which an interlayer jump has been carried out with the shortest distance is always a recorded region. Thus, there is a problem that, when the unrecorded region M is passed, the servo state becomes unstable.
In addition, if focusing unintentionally occurs in the unrecorded region M due to runaway or track deviation which occurs during an access or due to focus deviation, the signal levels differ depending on the recorded region R and the unrecorded region M as described above. Thus, track pull-in action cannot be carried out, and an operation of restoring to the recorded region R cannot be carried out smoothly.
In other words, as described above, in the conventional optical disk device, when an access to the recording region of an optical head is instructed, there is a case in which the unrecorded region M whose reflection index or the like differs from that of the recorded region R is passed when an access to a target position is made with the shortest distance. Thus, there is a problem that the servo state becomes unstable.